1. Field of the Invention
The present invention relates to a fine powder composition consisting of Mo-W solid solution suitable for use as a starting material for powder metallurgy, a process for the production thereof and a process for the production of a hard solid solution using the fine powder composition.
2. Description of the Prior Art
The hitherto known solid solution of (Mo-W) has been used as W-Mo alloy wires (ordinarily, 50% W-50% Mo, 30% W-70% Mo). Since a W-wire tends to be brittle at a high temperature, but its solid solution is soft and the temperature coefficient of resistance tends to suppress electric current, (Mo-W) wires have favourably been used. Tungsten base alloys can be improved in their properties as heat resisting materials by dissolving molybdenum therein in various proportions. Such alloys are, for example, W-Mo-Ta, W-Mo-Nb-C, W-Mo-V, etc. Tungsten and molybdenum belong to the same Group of the Periodic Table, have similar properties and are capable of forming solid solutions in any proportions, so the solid solutions of tungsten and molybdenum make up for the defects of each other and it would be possible to develop a new material therefrom. The density of molybdenum is about half that of tungsten and, as the price of tungsten has lately risen, the price of molybdenum is only half that of tungsten. Therefore, the use of (Mo-W) alloys or solid solutions as a substitute for tungsten articles will be increased at a time when tungsten articles are expensive.
Up to the present time, however, alloys composed of solid solutions of (Mo-W) have not so been developed except (Mo-W) wires, because tungsten and molybdenum have very high melting points and thus dissolving should be carried out at a temperature of 3000.degree. C. or higher so as to form such a solid solution. In 1952, Smithells proposed an electric current sintering method as a commercial method and substitute for the above described method. This method can be adapted to rod-shaped articles, but is not suitable for preparing tungsten articles and molybdenum articles having various shapes.
A tungsten powder produced by the prior art process for the production of tungsten powders contains a small amount of molybdenum (200 ppm) as an impurity, which tends to change the particle size of the tungsten powder and to change the electrical property of tungsten. Accordingly, in order to obtain the properties of high purity tungsten, many efforts have been made to decrease molybdenum in the refining step. Ores having a small content of molybdenum have been chosen and numerous studies have been made on a method of treating ores containing a large amount of molybdenum. However, none have proposed ideas whereby a large amount of molybdenum may be contained in tungsten, in opposition to the common knowledge.
It has hitherto been considered to be very difficult to powder (Mo-W) alloys, because these alloys are prepared by mixing molybdenum and tungsten, compacting and then subjecting to solid solution formation at a high temperature through the passage of electric current and sintering.
Up to the present time, as a starting material for cemented carbides, there has been used tungsten carbide (WC) powder as predominant component with a suitable binder metal, typically an iron group metal, to which carbides or carbonitrides of high melting point metals such as titanium, tantalum, niobium, molybdenum, hafnium, vanadium and chromium are added depending upon the requirements of a desired alloy. However, it is also true that tungsten is a relatively expensive metal and that it is found in only a few parts of the world. Accordingly, it is considered to be a so-called "strategic" material, and its availability can be subject to political considerations. Therefore, increase of the demand for cemented carbides consisting mainly of tungsten carbide meets inevitably with a problem of natural resources and if the tungsten carbide can be exchanged for another high melting point metal carbide, this exchange will have a great influence upon the industry.
Molybdenum monocarbide (MoC) is considered as a useful substitute, since this carbide only has the same crystal structure, a simple hexagonal type, as tungsten carbide, as well as mechanical properties similar to tungsten carbide. However, the existence of the hexagonal molybdenum monocarbide as a simple substance has remained in question to this date and thus an attempt to stabilize molybdenum has exclusively been carried out by forming a solid solution with tungsten carbide. This method was firstly reported by W. Dawihl in 1950, but this solid solution was not examined in detail and the commercial worth was not found in those days.
Of late, however, the study to utilize the solid solution (Mo.sub.x W.sub.y)C where x+y=1 has become active with the rise of the price of tungsten. It is very interesting why a study on this solid solution and an attempt to use the same has not been carried out so actively up to the present time.
In the prior art process for the production of a solid solution of MoC-WC, WC, Mo and C powders or W, Mo, C and Co powders are mixed, charged in a carbon vessel and reacted at a temperature of 1600.degree. to 2000.degree. C. (W. Dawihl: "Zeitschrift f. Anorganische Chemie" 262 (1950) 212). In this case, cobalt serves to aid in forming the carbide and to dissolve molybdenum and carbon in the tungsten carbide. In the absence of cobalt, it is very difficult to obtain a solid solution of (Mo, W)C. When a (Mo, W)C powder obtained by this method is used for the production of a cemented carbide alloy with a binder metal of cobalt as a substitute for WC, however, (Mo, W)C decomposes in the alloy to deposit needle crystals of (Mo, W).sub.2 C. Deposition of even a small amount of such a subcarbide with aggregation in the alloy causes deterioration of the strength of the alloy. For this reason, the use of MoC as a substitute for WC has not been attempted positively.
In a process for the production of mixed carbides, in general, carbides are heated in the presence of each other, optionally using a diffusion aiding agent such as cobalt, to give a uniform solid solution in most cases, but in the case of a composition of solid solution containing at least 70% of MoC, a uniform solid solution cannot be obtained by counter diffusion only at a high temperature. This is due to the fact that MoC is unstable at a high temperature and is decomposed into solid solutions such as (Mo, W)C.sub.1-x and (Mo, W).sub.3 C.sub.2 and, consequently, a solid solution (Mo, W)C of WC type cannot be formed only by cooling it. As a method of stabilizing this carbide, it has been proposed to react the components once at a high temperature, to effect diffusion of Mo.sub.2 C and WC, and to hold the product at a low temperature for a long time (Japanese Patent Application (OPI) No. 146306/1976). However, a considerably long diffusion time and long recrystallization time are required for forming (Mo, W)C from (Mo, W)C.sub.1-x and (Mo, W).sub.3 C.sub.2 at a low temperature. For the practice of this method on a commercial scale, the mixture should be heated for a long time in a furnace to obtain a complete carbide. This means that the productivity per furnace is lowered and a number of furnaces are thus required. When using a continuous furnace, on the other hand, a long furnace is necessary and mass production is difficult industrially.